Lightguides for signal transmission are generally fused silica glass optical fibers having a central core region surrounded by a cladding. The cladding has a lower index of refraction than the core. A single, continuous fiber may be many kilometers in length. Typically, the cladding is protected by a jacket of plastic material, such as an epoxy acrylate. It is desirable, especially for communications systems purposes, to be able to access the signal in a main lightguide fiber bus by coupling receiving and transmitting fiber buses to it to form an access port at any particular desired location along its length.
One type of access port is a microbend receiving structure. After removal of the outer jacket, the main fiber bus is bent locally at a sufficiently sharp angle to permit a small part of the signal to escape from the core through the cladding. The light is then picked up by a sensor which amplifies the signal for further use. The requirement for amplification of the received signal is a disadvantage, since it requires additional active equipment. Another disadvantage is that due to the poor coupling, it is not feasible to transmit a signal into a lightguide via a microbend port.
Another type of access port is made by removing the jacket from a segment of the main fiber bus and abrading or otherwise removing the cladding along the segment to form an optical flat surface which can be mated to similar surfaces of the receiving and transmitting fiber buses for optically coupling them to the main fiber bus. A disadvantage of this method is that the formation of the mating flat surfaces requires relatively time-consuming abrasion processes which are especially difficult to adapt to field use.